//
// third_party_lib.cpp
// ~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#include <boost/asio.hpp>
#include <array>
#include <iostream>
#include <memory>

using boost::asio::ip::tcp;

namespace third_party_lib
{

    // Simulation of a third party library that wants to perform read and write
    // operations directly on a socket. It needs to be polled to determine whether
    // it requires a read or write operation, and notified when the socket is ready
    // for reading or writing.
    class session
    {
    public:
        session(tcp::socket &socket)
            : socket_(socket)
        {
        }

        // Returns true if the third party library wants to be notified when the
        // socket is ready for reading.
        bool want_read() const
        {
            return state_ == reading;
        }

        // Notify that third party library that it should perform its read operation.
        void do_read(boost::system::error_code &ec)
        {
            if (std::size_t len = socket_.read_some(boost::asio::buffer(data_), ec))
            {
                write_buffer_ = boost::asio::buffer(data_, len);
                state_ = writing;
            }
        }

        // Returns true if the third party library wants to be notified when the
        // socket is ready for writing.
        bool want_write() const
        {
            return state_ == writing;
        }

        // Notify that third party library that it should perform its write operation.
        void do_write(boost::system::error_code &ec)
        {
            if (std::size_t len = socket_.write_some(
                    boost::asio::buffer(write_buffer_), ec))
            {
                write_buffer_ = write_buffer_ + len;
                state_ = boost::asio::buffer_size(write_buffer_) > 0 ? writing : reading;
            }
        }

    private:
        tcp::socket &socket_;
        enum
        {
            reading,
            writing
        } state_ = reading;
        std::array<char, 128> data_;
        boost::asio::const_buffer write_buffer_;
    };

} // namespace third_party_lib

// The glue between asio's sockets and the third party library.
class connection
    : public std::enable_shared_from_this<connection>
{
public:
    connection(tcp::socket socket)
        : socket_(std::move(socket))
    {
    }

    void start()
    {
        // Put the socket into non-blocking mode.
        socket_.non_blocking(true);

        do_operations();
    }

private:
    void do_operations()
    {
        auto self(shared_from_this());

        // Start a read operation if the third party library wants one.
        if (session_impl_.want_read() && !read_in_progress_)
        {
            read_in_progress_ = true;
            socket_.async_wait(tcp::socket::wait_read,
                               [this, self](boost::system::error_code ec)
                               {
                                   read_in_progress_ = false;

                                   // Notify third party library that it can perform a read.
                                   if (!ec)
                                       session_impl_.do_read(ec);

                                   // The third party library successfully performed a read on the
                                   // socket. Start new read or write operations based on what it now
                                   // wants.
                                   if (!ec || ec == boost::asio::error::would_block)
                                       do_operations();

                                   // Otherwise, an error occurred. Closing the socket cancels any
                                   // outstanding asynchronous read or write operations. The
                                   // connection object will be destroyed automatically once those
                                   // outstanding operations complete.
                                   else
                                       socket_.close();
                               });
        }

        // Start a write operation if the third party library wants one.
        if (session_impl_.want_write() && !write_in_progress_)
        {
            write_in_progress_ = true;
            socket_.async_wait(tcp::socket::wait_write,
                               [this, self](boost::system::error_code ec)
                               {
                                   write_in_progress_ = false;

                                   // Notify third party library that it can perform a write.
                                   if (!ec)
                                       session_impl_.do_write(ec);

                                   // The third party library successfully performed a write on the
                                   // socket. Start new read or write operations based on what it now
                                   // wants.
                                   if (!ec || ec == boost::asio::error::would_block)
                                       do_operations();

                                   // Otherwise, an error occurred. Closing the socket cancels any
                                   // outstanding asynchronous read or write operations. The
                                   // connection object will be destroyed automatically once those
                                   // outstanding operations complete.
                                   else
                                       socket_.close();
                               });
        }
    }

private:
    tcp::socket socket_;
    third_party_lib::session session_impl_{socket_};
    bool read_in_progress_ = false;
    bool write_in_progress_ = false;
};

class server
{
public:
    server(boost::asio::io_context &io_context, unsigned short port)
        : acceptor_(io_context, {tcp::v4(), port})
    {
        do_accept();
    }

private:
    void do_accept()
    {
        acceptor_.async_accept(
            [this](boost::system::error_code ec, tcp::socket socket)
            {
                if (!ec)
                {
                    std::make_shared<connection>(std::move(socket))->start();
                }

                do_accept();
            });
    }

    tcp::acceptor acceptor_;
};

int main(int argc, char *argv[])
{
    try
    {
        if (argc != 2)
        {
            std::cerr << "Usage: third_party_lib <port>\n";
            return 1;
        }

        boost::asio::io_context io_context;

        server s(io_context, std::atoi(argv[1]));

        io_context.run();
    }
    catch (std::exception &e)
    {
        std::cerr << "Exception: " << e.what() << "\n";
    }

    return 0;
}